Brushless direct current (BLDC) motors are used in industries such as appliances, automotive, aerospace, consumer, medical, industrial automation equipment and instrumentation. BLDC motors do not use brushes for commutation, instead, electronic commutation is used. BLDC motors have advantages over brushed DC motors and induction motors such as: better speed versus torque characteristics, higher dynamic response, higher efficiency, longer operating life, longer time intervals between service, substantially noiseless operation, and higher speed ranges. A more detailed synopsis of BLDC motors may be found in Microchip Application Note AN857, entitled “Brushless DC Motor Control Made Easy;” and Microchip Application Note AN885, entitled “Brushless DC (BLDC) Motor Fundamentals;” both at www.microchip.com, and both are hereby incorporated by reference herein for all purposes.
BLDC motor control requires three things: (1) pulse width modulation (PWM) drive voltages to control the motor speed, (2) a mechanism to commutate the stator of the BLDC motor, and (3) a way to estimate the rotor position of the BLDC motor. PWM may be used to provide a variable voltage to the stator windings of the BLDC motor for speed control thereof. The effective voltage provided thereto is proportional to the PWM duty cycle. The inductances of the stator coils act as low pass filters to smooth out the PWM voltages to substantially direct current (DC) voltages. When properly commutated, the torque-speed characteristics of a BLDC motor are substantially identical to a DC motor. The PWM derived variable voltage controls the speed of the motor and the available torque.
A three-phase BLDC motor completes an electrical cycle, i.e., 360 electrical degrees of rotation, in six steps at 60 electrical degrees per step. Synchronously at every 60 electrical degrees, winding phase current switching is updated (commutation). However, one electrical cycle may not correspond to one mechanical revolution (360 mechanical degrees) of the motor rotor. The number of electrical cycles to be repeated to complete one mechanical revolution depends upon the number of rotor pole pairs.
BLDC motors are not self-commutating and therefore are more complicated to control. BLDC motor control requires knowledge of the motor rotor position and a mechanism to commutate the BLDC motor stator windings. For closed-loop speed control of a BLDC motor there are two additional requirements, measurement of rotational speed and a pulse width modulation (PWM) drive signal to control the motor speed and power therefrom.
To sense the rotor position of the BLDC motor, Hall Effect sensors may be used to provide absolute rotor position sensing. However, Hall Effect sensors increase the cost and complexity of a BLDC motor. Sensor-less BLDC control eliminates the need for Hall Effect sensors by monitoring the back electromotive force (BEMF) voltages at each phase (A-B-C) of the motor to determine drive commutation. The drive commutation is synchronized with the motor when the BEMF of the un-driven phase crosses one-half of the motor supply voltage in the middle of the commutation period. This is referred to as “zero-crossing” where the BEMF varies above and below the zero-crossing voltage over each electrical cycle. Zero-crossing can only be detected on the un-driven phase when the drive voltage is being applied to the other two driven phases. So detecting a change of the BEMF on the un-driven phase from less than to greater than one-half of the motor supply voltage may be used during application of the drive voltage to the two driven phases for a three phase BLDC motor.
Controlling brushless DC (BLDC) motors can be challenging in particular if a BLDC motor does not provide for any sensors that are capable to determine a current position of the rotor. Hence, there is a need for a universal zero-cross detection for sensor-less BLDC motor control. One way of determining a current rotor position in BLDC applications is to use back electromotive force (BEMF) signals, which is the voltage, or electromotive force, that pushes against the current which induces it. BEMF is the voltage drop in an alternating current (AC) circuit caused by magnetic induction. Generally, when driving a BLDC motor, for example through two of its three coils of a three phase BLDC motor, BEMF signals can be received through the unused coil. BEMF signals vary in amplitude and position relative to the PWM signal that is applied to the driven coils. Zero crossing detection of these BEMF signals can be sampled at specific times, wherein commutation occurs at the mid-point between the zero crossing detections. However, these samples may be corrupted by the motor characteristics. Therefore, for proper flux integration the motor characteristics must be known, and the drive and motor voltages must also be in phase. Using field oriented control (FOC) requires high speed analog-to-digital converters (ADCs), computations are math intensive, e.g., high processing power required, and a non-FOC method has to be used to start the motor (BEMF measurements require the motor rotor to be turning).